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Understanding SMF Security

From Online Manual

SMF is proud of being one of the most secure forum systems in the world. Of course this was not done magically, SMF has strict rules and practices to achieve security. Here is some guidelines to understand the main and most important techniques for SMF-level security.

Permissions

SMF relies on a powerful permission system. As a mod writer, you should already know how you can create and check for permissions. Permissions contribute the security by providing some functions to some member(group)s, and not to others. You should be careful when you are checking permissions.

Permissions are to be used before displaying the action page. Before displaying a page that needs some permissions, first check the permission and then display the page.

Permissions are also to be checked before the action is taken. When your mod displayed a page, the user enters some values to the form or clicks some links. When the user completes his action, before transferring the data to database (or processing any other action), you MUST check the permissions again. This is more important than checking the permission before displaying the page.

Session Checks

Session checking is one of the two most important practice to make SMF secure. It is for preventing malicious users to make other users do something that they originally didn't intend to.

What is session checking?

There are two types of links in SMF. One is to "open a window" to do something, the other is to "do something" directly.

For example, clicking on the "move" button of a topic opens a new window before doing anything. It asks you the board you want to move and the redirection topic. When you click on a "move topic" link, nothing happens but it displays a new screen. No actual movement is processed.

But clicking "lock" button for a topic is different. It directly locks the topic. The admin is not displayed a new confirmation screen, but the topic is locked directly.

This second case is what you should be careful about. For example, you have created a modification, which is used to increase other users' karma points by 10. You used this link as the karma increase link:

forum.com/index.php?action=karma;sa=increase;u=156

This link is supposed to increase the karma of user ID 156, and it works like a charm. When a user clicks the link, user 156 directly gains 10 more karma. But this link is insecure!

Because, it is a direct link. A malicious user can exploit it to increase his karma by thousands! He can do two things:

Put a fake link: www.google.com , and any user clicking on it will have accidentally increased 156's karma.

Request a php image in the post: [img]forum.com/index.php?action=karma;sa=increase;u=156[/img] , which will be displayed to the users each time they visit the page. And this image code will request the target page each time, increasing 156's karma rapidly.

So, in order to prevent fooling of an admin/user by an external call of such a link, a "sesc=<PHPSESID>" is appended after these links. Like:
forum.com/index.php?action=karma;sa=increase;u=156;sesc=d113eb64c27777ca1037d8c0ff51ae92

In this case, everyone has a unique link to increase somebody's karma. This sesc is different for every member, also it changes everytime the users log in. So inserting a fake hyperlink won't work: www.google.com. This link only works is a user with session ID d113eb64c27777ca1037d8c0ff51ae92 clicks on the link.

To note: there is no way someone can "guess" other users' session IDs. So, your link is secured. (Of course you should also learn [#post_howsesc" class="bbc_link">how to "check" for session ids].)

When do you need session checking?

As said, you should use session checking before doing a direct action.
If the link opens a new window before actually doing something, there is no need for session checking.

Here are some examples:
(These links open a new window before doing something)

Move topic

Merge topics

Delete user (in profile view)

Post & Reply

(These links DO their job without opening a window)

Lock topic

Remove topic

Sticky topic

Mark As Read

Notify

You should try and see that these links really have that session IDs (or do not have).

Verifying the sessions will also be [#post_howsc" class="bbc_link">described below].

What is form sessions?

There is a second type session checking in SMF. This is applied when someone inputs and posts a form, like a post form or a profile editing form.

When a user posts a form, you should be sure that the form is actually taken from the correct page. A form can easily be fooled by a malicious user.

Say, you are a malicious user. You want to become an admin. You know that the actual admin needs to change your membergroup from your profile, and then hit "save". When he hits "save", a form data is submitted to SMF.

You create a webpage on another website. It includes hidden form elements, including a new membergroup data. And the page also has a form button.

Then you convince the admin to click on it in some way. Like "hey, look at htt======p://mycoolsite.com". When the admin goes to the page, he sees two buttons: "I'm over 18" and "I'm under 18". He clicks either one of them. And, a form is submitted!

The form target was actually the SMF site itself. When the admin clicked the button, he sent a form to his own forum accidentally, which was designed to make you an admin!

ALL forms must include session checks in it! There are NO forms that do not need session verification!

How do I secure the forms?

Simple: form sessions. It is line URL sessions. You put a session ID value to the form.
<input type="hidden" name="sc" value="d113eb64c27777ca1037d8c0ff51ae92" />
When the form is submitted by the user, SMF will check whether a correct value of session was recieved or not. If not recieved, SMF won't do that function.

Since the malicious user can not guess the correct value of the session ID of the admin, he can not create a fake form.

How do I apply session checking into my code?

Applying URL sessions

When designing templates or links to be displayed to the users, you should append "sesc=<PHPSESSID>" to the links. This is achieved by this simple code:
sesc=' . $context['session_id'] . '

Make sure you verify that the URL really has the session ID in it now.

Applying form sessions

You should be sure that the form contains a hidden element between <form> and </form> tags. It is usually inserted right before or after the "save" button.
This element is <input type="hidden" name="sc" value="d113eb64c27777ca1037d8c0ff51ae92" />, which can be inserted like this:

Checking form sessions

Verifying that a form has a correct session is as simple as the URL case. You use
checkSession();
to see that the form data has the hidden element "sc" in it and it is correct.

Example:

// Assuming that you have already verified that $userID and $groupID is valid,
function ChangeMemberGroup($userID, $groupID)
{
global $db_prefix;
isAllowedTo('manage_members'); // Always do the permission check!
checkSession(); // Verify that the form has a "sc" element in it.
// Do the actual job
db_query("
UPDATE {$db_prefix}members
SET ID_GROUP = $groupID
WHERE ID_MEMBER = $userID
", __FILE__, __LINE__);
}

checkSession(); function will

check whether the form has a "sc" element in it,

do nothing if the sc is correct,

throw a "session verification failed" error if the form:

does not have a sc,

has an incorrect sc.

After you used these session verification systems, your forms and links are more secure. Do not forget that:

ALL directs link to do a job should have "sesc" in it. The links that opens a new window before actually doing a job won't need "sesc" in it.

ALL form should have "sc" elements in it.

A session ID of a user can not be guessed by another user.

Input Validations

Input validation is the other very important security practice that a mod MUST use.

What is input validation?

Input validation is to check whether a data sent to SMF is actually the data that was expected. For example, you made a mod to increase the karma of a user by 10. The increasing URL has the user ID.
forum.com/index.php?action=karma;sa=increase;===u=156===;sesc=d113eb64c27777ca1037d8c0ff51ae92

"OR 1" will result in an increase of everyone's karma, not only the user 156. This might not look that malicious, but there are cases it is.

Consider this:

You have a mod, a profile field that is stored in members table. It is asking you the name of your favorite game. You are sending the data using a form within your profile. And your form sends $_POST['favorite_game'] as the input.

The last one is defined by SMF itself. It is similar to htmlspecialchars_decode of php. Look for the function definitions of these at php.net to get more information.

Always have a look at SMF core codes itself, to decide where you must use validation and how. For example, see Profile.php and track the use of validation functions used on a profile field variable until it is inserted to the database.

Of course there are more security measures that you must take when creating mods, but these are the fundamental ones widely used by SMF itself.